Polyamide has excellent toughness owing to a hydrogen bond of an amide group, and is used for mechanical parts, building materials, films, or fibers. When polyamide is produced, a method for polymerizing a ring-opening polymerizable monomer containing an amide bond is used. In the case where Nylon 6 is produced as one example of the polyamide, for example, known is a method where ε-caprolactam, which is a ring-opening polymerizable monomer containing an amide bond, is polymerized through melt polymerization in the presence of a water-catalyst.
In the case of melt polymerization using a water-catalyst, however, a reaction speed is slow, and it may take a few hours to complete a polymerization reaction. In accordance with this method, moreover, it is necessary to carry out the reaction at temperature equal to or higher than a melting point of a resulting polymer, and about 10% of the ring-opening polymerizable monomer and oligomer is remained in the polymer product due to an equilibrium reaction. Therefore, there is a case where a step for removing the ring-opening polymerizable monomer or oligomer from the polymer product for industrial use.
Moreover, also known is a method where ε-caprolactam is polymerized through ring-opening polymerization by combining melt polymerization and solid phase polymerization (see PTL 1). In the case where this method is used, however, a polymerization reaction takes a few hours, and a residual amount of the ring-opening polymerizable monomer and oligomer in the polymer product is large.
In order to shorten a reaction time and reduce a residual amount of a ring-opening polymerizable monomer and oligomer in production of polyamide, an anionic polymerization method has been used. In the case where this method is used, polyamide is produced by polymerizing a ring-opening polymerizable monomer containing an amide bond using an alkali metal (e.g., sodium, and potassium) or a Grignard reagent as a main catalyst. In accordance with this method, a polymer having a high molecular weight can be obtained with a short reaction time due to high reactivity. In accordance with this method, moreover, a reaction can be carried out at low temperature compared to a melt polymerization method, and therefore an amount of the ring-opening polymerizable monomer and oligomer remained in the polyamide product can be reduced.
Disclosed as an example of an anionic polymerization method is a method where ε-caprolactam is polymerized using a phosphazene basic compound and N-acetyl-ε-caprolactam as catalysts (see PTL 2). Moreover, disclosed is to attain a polymer having the weight average molecular weight of 780,000 polymerization of anhydrous ε-caprolactam performed for 20 minutes in the presence of ethylmagnesium bromide as a Grignard compound, and adipoyl biscaprolactam as an acyl lactam compound (see PTL 3).
In the case where an anionic polymerization method is used in production of polyamide, a generated product is solidified in the second half of the reaction, as the reaction temperature is set to the temperature equal to or lower than the melting point of the polymer product (for example, 150° C. or lower in case of Nylon 6). For example, it is disclosed that fluidity of a system is lost and a polymer product is solidified 18 minutes later from an initiation of polymerization, when the polymerization is initiated at 150° C. by adding N-acetylcaprolactam and ethylmagnesium bromide to ε-caprolactam (see PTL 4).